Effects of light on NO 3− uptake in small forested streams: diurnal...

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Effects of light on NO3- uptake in small forested streams diurnal and day-to-day

variationsAuthor(s) Patrick J Mulholland Steven A Thomas H Maurice Valett Jackson R Webster and JakeBeaulieuSource Journal of the North American Benthological Society 25(3)583-595 2006Published By The Society for Freshwater ScienceDOI httpdxdoiorg1018990887-3593(2006)25[583EOLONU]20CO2URL httpwwwbiooneorgdoifull1018990887-359328200629255B5833AEOLONU5D20CO3B2

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J N Am Benthol Soc 2006 25(3)583ndash595 2006 by The North American Benthological Society

Effects of light on NO3 uptake in small forested streams diurnal

and day-to-day variations

Patrick J Mulholland1

Environmental Sciences Division Oak Ridge National Laboratory PO Box 2008 Oak RidgeTennessee 37831 USA

Steven A Thomas2 H Maurice Valett3 AND Jackson R Webster4

Department of Biology Virginia Polytechnic Institute and State University Blacksburg Virginia 24061 USA

Jake Beaulieu5

Department of Biological Sciences University of Notre Dame Notre Dame Indiana 46556 USA

Abstract We investigated the effects of autotrophy on short-term variations in nutrient dynamics bymeasuring diurnal and day-to-day variations in light level primary productivity and NO3

uptake duringearly and late spring in 2 forested streams the East and West Forks of Walker Branch in eastern TennesseeUSA We predicted that diurnal and day-to-day variations in NO3

uptake rate would be larger in the WestFork than in the East Fork in early spring because of higher rates of primary productivity resulting from amore stable substratum in the West Fork We also predicted minimal diurnal variations in both streams inlate spring after forest leaf emergence when light levels and primary productivity are uniformly low Reach-scale rates of gross primary production (GPP) were determined using the diurnal dissolved O2 changetechnique and reach-scale rates of NO3

uptake were determined by tracer 15N-NO3 additions In the West

Fork significant diurnal and day-to-day variations in NO3 uptake were related to variations in light level

and primary productivity in early spring but not in late spring consistent with our predictions In earlyspring West Fork NO3

uptake rates were 2 to 33 higher at midday than during predawn hours and 50higher on 2 clear days than on an overcast day several days earlier In the East Fork early spring rates ofGPP were 4 to 53 lower than in the West Fork and diurnal and day-to-day variations in NO3

uptake rateswere 30 considerably lower than in the West Fork However diurnal variations in NO3

uptake rateswere greater in late spring in the East Fork possibly because of diurnal variation in water temperature Ourresults indicate the important role of autotrophs in nutrient uptake in some forested streams particularlyduring seasons when forest vegetation is dormant and light levels are relatively high Our results also haveimportant implications for longer-term assessments of N cycling in streams that rely on daytimemeasurements or measurements only under limited weather conditions (ie clear days)

Key words nitrate uptake light diurnal patterns tracer 15N gross primary production nutrientspiraling

The role of autotrophs in the structure and function-

ing of streams has been an important topic in stream

ecology for some time In his seminal paper Minshall

(1978) argued that autotrophy is of primary impor-

tance in maintaining the structure and function of

many streams including those in forested catchments

that may have high rates of primary productivity for

only a few months each year In their interbiome

comparison of stream metabolism Bott et al (1985)

showed that rates of gross primary production (GPP)

exceeded respiration in one or more seasons in most

streams even those in forested regions

A number of studies have focused on the role of

autotrophy in total metabolism and support of the

food web in streams (Bott et al 1985 Hill et al 1995

1 E-mail address mulhollandpjornlgov2 Present address School of Natural Resources 309

Biochemistry Hall University of Nebraska LincolnNebraska 68583 USA E-mail sthomas5unledu

3 E-mail addresses mvalettvtedu4 jwebstervtedu5 jbeauliendedu

583

Lamberti 1996) There also has been interest in the roleof autotrophs in nutrient uptake and cycling instreams particularly in recent years Grimm (1987)showed that rates of dissolved inorganic N (DIN)uptake increased during algal regrowth followingflash floods in an Arizona stream resulting indeclining DIN concentrations in stream water Sabateret al (2000) found that uptake rates of PO4

3 (but notNH4

thorn) were highly correlated with rates of primaryproduction in a study comparing streams with loggedand unlogged riparian forests in Spain Hall and Tank(2003) reported that 75 of the variation in NO3

uptake rate was explained by variation in rates of GPPin a study of N uptake and metabolism in streams inthe Grand Teton National Park McKnight et al (2004)found higher rates of nutrient uptake by algae andlower N and P concentrations in streams where algalmats were abundant than where they were sparse intheir study of Antarctic streams Controls on Ntransformations in streams are of particular interestbecause N availability is increasing rapidly because ofhuman activities (Vitousek et al 1997) and streams arehot spots of N uptake and retention within landscapes(Alexander et al 2000 Peterson et al 2001)

Seasonal variation in primary production can resultin similar variation in nutrient uptake in streams Inforested regions this variation is related to the leafphenology of riparian vegetation In the West Fork ofWalker Branch eastern Tennessee USA analysis oflong-term data records has indicated consistent sea-sonal changes in nutrient concentrationsmdashdecline inconcentrations during late winter and early spring andsubsequent increases in concentrations in late springattributable to changes in instream uptake rates drivenby leaf emergence in the riparian forest canopy(Mulholland and Hill 1997 Mulholland 2004) Hill etal (2001) showed strong relationships between lightlevel periphyton photosynthesis streamwater nu-trient concentrations and growth of the dominantherbivore in Walker Branch and a nearby forestedstream during spring indicating a tight cascade ofshade effects through primary producers to biotic(food chain) as well as abiotic (nutrients) componentsof the ecosystem

In addition to seasonal variations short-term varia-tions in nutrient uptake in streams may be caused bydiurnal or day-to-day variations in light level andprimary production These light-driven short-termvariations in uptake may be particularly evident inthe case of NO3

because energy is required for itsreduction prior to its use in cellular synthesis Severalprevious studies have reported diurnal variations inNO3

concentration in streams with minimum con-centrations coinciding with maximum rates of GPP at

midday (Manny and Wetzel 1973 Grimm 1987Mulholland 1992 Burns 1998) These studies suggestautotroph-driven variation in NO3

uptake in streamseven those draining forested catchments

We investigated the effects of diurnal and day-to-day variations in light level on NO3

uptake duringearly and late spring in 2 forested streams the Eastand West Forks of Walker Branch Previous researchindicated that the early spring peak in primaryproduction in the East Fork is considerably lowerthan that in the West Fork (Mulholland et al 2000PJM unpublished data) probably because of differ-ences in substrata Therefore we predicted thatdiurnal and day-to-day variations in NO3

uptakewould be more prominent in the West Fork than in theEast Fork in early spring and that diurnal and day-to-day variations would be minimal in both streams inlate spring after forest leaf emergence when lightlevels and primary productivity are uniformly low Aprevious study in the East Fork during summerindicated that day and night uptake of NO3

differed(Fellows et al 2006) however this study relied onchamber incubations of benthic substrata and may notreflect reach-scale processes Our study used a fieldtracer 15N addition approach to quantify diurnal andday-to-day variations in NO3

uptake at the stream-reach scale

Study Sites

The study was conducted in the West and East Forksof Walker Branch Watershed (lat 35858 0N long848170W) a deciduous forest watershed in the USDepartment of Energyrsquos Oak Ridge EnvironmentalResearch Park in the Ridge and Valley region ofeastern Tennessee Both streams are 1st order andoriginate as springs 100 to 200 m upstream from thestudy reaches Mean annual precipitation is 140 cmand mean annual temperature is 1458C The water-sheds of both streams are underlain by several layersof siliceous dolomite and stream water is slightly basicThe substratum of the West Fork is primarily cobbleand bedrock outcrops whereas the East Fork sub-stratum is primarily gravel and fine-grained organic-rich sediments These substratum differences are theresult of differences in stratigraphy of the underlyinggeology (Knox dolomite) and both are typical ofstreams in the Ridge and Valley Province of easternTennessee (Johnson and Van Hook 1989) Streamgradients are relatively low 0035 for the West Forkand 0020 for the East Fork More detailed descriptionsof these streams are given by Mulholland et al (2000)and Mulholland et al (2004)

584 [Volume 25P J MULHOLLAND ET AL

Methods

15N addition

Two series of tracer 15N addition experiments wereconducted in each stream one during the early springbefore leaf emergence (5ndash9 April 2001) and the otherduring late spring well after leaf emergence (11ndash12June 2001) Each experiment consisted of a continuousinjection of 99 15N-enriched KNO3 and a conserva-tive tracer (NaCl) for 5 to 22 h to each stream andmeasurement of 15N-NO3

and Cl concentrations at 2stations downstream from the injection after steadystate was achieved The upper measurement station inboth streams was 10 m downstream from the 15Ninjection a distance long enough for complete mixingof the tracer The lower measurement stations were 120m downstream from 15N injection in the West Fork and90 m downstream in the East Fork The K15NO3 andNaCl tracers were dissolved in carboys containing 15L of distilled water and pumped into the streams usinga battery-powered fluid metering pump (FMI SyossetNew York) The amount of K15NO3 and NaCl added tothe carboy for each injection varied depending onstream discharge and ambient NO3

concentration Ineach injection addition of K15NO3 increased the15N14N ratio of streamwater NO3

by 203 relativeto the ambient ratio and resulted in only a small (7)increase in NO3

concentration Addition of NaClincreased the streamwater Cl concentration by 10 to15 mgL

In April the 15N injections in each stream werebegun at 2000 h on 4 April Stream samples werecollected just before the injections (background meas-urements) and during the injections at 2400 h(midnight) on 4 April and 0600 h (predawn) and1400 to 1500 h (midday) on 5 April The 15N injectionswere terminated after the midday sampling becauselight levels were relatively low from overcast weatherconditions Additional 15N injections were done on 7April and 9 April (the latter only in the West Fork)under mostly clear weather conditions These 15Ninjections were begun at 0900 to 1000 h and streamsamples were collected between 1400 and 1500 h(midday)

In June the 15N injections in each stream were begunat 2000 h on 11 June Stream samples were collectedjust before the injections and during the injections at2400 h (midnight) on 11 June and 0500 to 0600 h(predawn) 1000 h (midmorning) and 1300 to 1400 h(midday) on 12 June The 15N injections wereterminated after the midday sampling In June 15Ninjections were not done on different days as in Aprilbecause light levels beneath the forest canopy werelow and did not vary much from day to day

Water temperature was measured and 4 replicatewater samples (2 L each) were collected from theupper and lower sampling stations during eachsampling period All samples were immediatelyfiltered in the field (Whatman no 1 cellulose nominalpore size frac14 11 lm) and 1-L (for analysis of 15N-NO3

)and 30-mL (for analysis of NO3

and Cl concen-trations) subsamples of the filtrate were returned to thelaboratory within 2 h of collection

Photosynthetically active radiation (PAR) also wasmeasured throughout each experimental period at onelocation in each stream using a quantum sensor (LiCor190SA LiCor Lincoln Nebraska) and data logger(Campbell Scientific CR-10 Campbell Scientific Lo-gan Utah)

Laboratory analyses

Cl concentration was measured by ion chromatog-raphy and NO3

concentration was measured byautomated CundashCd reduction followed by azo-dyecolorimetry (Bran Luebbe Auto Analyzer 3 SealAnalytical Mequon Wisconsin APHA 1992)

Additions (spikes) of unlabelled KNO3 (200 lg NL)were made to 1-L samples for 15N-NO3

analysis toreduce 15N14N ratios to the ideal working range formass-spectrometric measurement Identical spikes alsowere added to 1-L samples of deionized water tocalculate N recovery and to determine the 15N14Nratio of the NO3

spike The NO3 measurement was

actually NO3 thorn NO2

but NO2 was assumed to be

negligible in this well-oxygenated stream (Mulholland1992) Concentrations of NO3

were expressed interms of N (eg lg NL)

Processing of samples for 15N-NO3 analysis was

modified from the method of Sigman et al (1997)Samples ranging in volume from 005 to 1 L (depend-ing on NO3

concentration) were added to glass flaskscontaining 5 g of NaCl and 3 g of MgO Deionizedwater was added to small samples (high NO3

concentrations) to bring the initial sample volume to200 mL The samples were then brought to a gentleboil on a hot plate until the volume was reduced to100 mL thereby concentrating 15NO3

and degass-ing NH3 produced from NH4

thorn under alkaline con-ditions The concentrated samples were then cooledtransferred to 250-mL high-density polyethylene bot-tles and refrigerated until further processing The15NO3

in the concentrated samples was captured foranalysis using a reductiondiffusionsorption proce-dure as follows To start 05 g of MgO and 3 g ofDevardarsquos alloy were added to each sample to reduceall NO3

to NH3 under alkaline conditions Immedi-ately afterward a filter packet consisting of aprecombusted acidified (25 lL of 25-molL KHSO4)

2006] 585NO3

UPTAKE IN STREAMS

glass-fiber filter (1-cm Whatman GFD) sealed between2 Teflont filters (Millipore white nitex LCWP 25-mmdiameter 10-lm pore size) was placed in each sampleand floated to absorb the liberated NH3 Parafilmt wasplaced over the mouth of each sample bottle and thebottle was tightly capped Samples were then heatedto 608C for 2 d and shaken at room temperature for anadditional 7 d to allow full reduction of NO3

to NH4thorn

conversion of NH4thorn to NH3 diffusion of NH3 into the

sample headspace and absorption of NH3 onto theGFD filter At the end of this incubation period filterpackets were removed from the sample bottles anddried in a desiccator for 2 d after which the Teflonfilter packets were opened and the GFD filtersremoved Each GFD filter with its absorbed NH3 wasencapsulated in a 5 3 9-mm aluminum tin and placedin a 96-well titer plate with each well capped Allsamples were sent to the stable isotope laboratory atthe University of Waterloo Waterloo Ontario (httpwwwscienceuwaterloocaresearcheilabAboutinContentContenthtml) for 15N14N ratio analysis bymass spectrometry using a Europa Integra continuousflow isotope-ratio mass spectrometer coupled to an in-line elemental analyzer for automated sample com-bustion (SerCon LTD Crewe UK)

Measurements of 15N14N ratio were expressed asd15N values () according to the equation

d15N frac14 RSAMPLE

RSTANDARD

1

3 1000 frac121

where RSAMPLE is the 15N14N ratio in the sample andRSTANDARD is the 15N14N ratio in atmospheric N2

(RSTANDARD frac14 00036765)

Calculation of tracer 15N flux

Tracer 15N flux was calculated from the measuredd15N values in a series of steps described in Mulhol-land et al (2004) First d15N values were converted to15N(15N thorn 14N) ratios using the equation

15N15Nthorn14N

frac14

d15N

1000thorn 1

3 00036765

1thorn d15N

1000thorn 1

3 00036765

frac122

where 15N(15N thorn 14N) is the atom ratio (AR) of 15N15NO3

AR values were corrected for the addedNO3

spike using the equation

ARi frac14ethfrac12NO3Ni thorn frac12NO3 NspTHORNethARmiTHORN ethfrac12NO3 NspTHORNethARspTHORN

frac12NO3 Nifrac123

where [NO3Ni] is the measured NO3

concentrationat station i (lg NL) [NO3

Nsp] is the increase inNO3

concentration in the water sample resulting fromthe NO3

spike (lg NL same for all stations) ARmi isthe AR value at station i calculated from the measuredd15N values on spiked samples from station i usingequation 2 ARsp is the AR value of the NO3

spikecalculated from the measured d15N values of NO3

inthe deionized water samples that also received theNO3

spike and ARi is the true AR value of NO3 at

station i Background-corrected AR values were thencomputed at each station i (ARbci) by subtracting thebackground AR values (ARb calculated from themeasured d15N values at the upstream station usingequation 2) from the ARi values calculated for samplescollected at the stations downstream from the 15Ninjection as

ARbci frac14 ARi ARb frac124

Last the tracer 15NO3 mass flux at each station i

(15Nflux i lg Ns) was computed by multiplying ARbci

by the streamwater NO3 concentration ([NO3-Ni])

and stream discharge (Qi) at each station i as

15Nflux bci frac14 ARbcifrac12NO3 NiQi frac125

Qi at each station was determined from the increase instreamwater Cl concentration during the injection as

Qi frac14 ethfrac12ClinjQpumpTHORN=ethfrac12Cli frac12ClbTHORN frac126

where the Cl injection rate (mgs) was calculated asthe product of the Cl concentration in the injectionsolution ([Clinj]) and the solution injection rate (Qpump)and the increase in Cl concentration at each station iwas calculated as the difference between Cl concen-tration during the injection ([Cli]) and the measuredCl concentration just before the 15N injection (iebackground concentration [Clb])

Calculation of NO3 uptake parameters

The total uptake rate of NO3 expressed as a

fractional uptake rate from water per unit distance(k m) was calculated for each sampling period froma single regression of ln(tracer 15NO3

flux) vs distance(Newbold et al 1981 Stream Solute Workshop 1990)The inverse of k is the uptake length of NO3

Errorassociated with the calculated values of k wasestimated as the error in each regression slope basedon the 8 measurements made in each stream (4measurements at each of 2 locations) This approachfor determining k and its error using measurements atonly 2 stations does not include error associated withlongitudinal variation in uptake rate but it does allowstatistical comparisons of k values for the same reach


in each stream for different sampling periods (iedifferent times of the day or on different dates)Statistical differences in k between sampling periodswere determined using the SAS General Linear Modelsprocedure (version 82 SAS Institute Cary NorthCarolina)

Uptake velocity (Vf) was calculated from k using theequation

Vf frac14 kud frac127

where u is the average water velocity and d is theaverage water depth (Stream Solute Workshop 1990)Total NO3

uptake was also calculated as a massremoval rate from water per unit area (U lg N m2

min1) using the equation

U frac14 Fk

wfrac128

where F is the average flux of NO3 (as N) in

streamwater in the experimental reach (determinedas the product of average NO3

concentration andaverage discharge) and w is the average stream wettedwidth (Newbold et al 1981) Error in Vf and U wasestimated from error in k assuming no error inmeasurements of NO3

concentration discharge ud and w

Whole-stream metabolism measurements

Whole-stream rates of GPP and total respiration (R)were determined using the upstreamndashdownstreamdiurnal dissolved O2 change technique (Marzolf et al1994) with the modification suggested by Young andHuryn (1998) for calculating the airndashwater exchangerate of O2 Measurements of dissolved O2 concen-tration and water temperature (YSI 6000 series sondesYSI Yellow Springs Ohio) were made at 5-minintervals at the 2 sampling stations in each streamover each of the experimental periods Exchange of O2

with the atmosphere was calculated based on theaverage O2 saturation deficit or excess within thestudy reach and the reaeration rate determined fromthe decline in dissolved propane concentration duringsteady-state field injections of propane and a con-servative tracer (Cl to account for dilution of propaneby groundwater inflow) done during the measurementperiod in each stream The reaeration rate of propanewas converted to O2 using a factor of 139 (Rathbun etal 1978) The net rate of O2 change caused bymetabolism (equivalent to net ecosystem production[NEP]) was then calculated at 5-min intervals from thechange in mass flux of dissolved O2 between stationscorrected for airndashwater exchange of O2 within thereach

The daily rate of R was calculated by summing thenet O2 change rate measured during the night and thedaytime rate of R determined by a linear extrapolationbetween the net O2 change rate during the 1-hpredawn and postdusk periods The daily rate ofGPP was determined by summing the differencesbetween the measured net O2 change rate and theextrapolated value of R during the daylight period Allmetabolic rates were converted to rates per unit areaby dividing by the area of stream bottom between the2 stations (determined from the measurement ofwetted channel width at 1-m intervals over eachreach)

Results

Physical and chemical conditions

Physical and chemical conditions were generallysimilar during 15N addition experiments in the samestream and month (Table 1) During the April experi-ments before leaf emergence in the forest canopyNO3

concentrations in each stream were slightlyhigher in predawn samples than in samples taken atother times Water temperatures were somewhathigher during midday sampling than during nightsampling in both streams particularly on the high-light dates Discharge declined slightly during thesequential experiments in each stream Discharge alsowas lower in June than in April as is typical for thesestreams because of high evapotranspiration ratesduring the growing season

Metabolism and biomass

The daily PAR fluxes to both streams were low on 5April because of extensive cloud cover but increasedsubstantially on 7 April and 9 April under mostly clearweather conditions (Table 2) Daily PAR values wereconsiderably lower on 12 June after full leaf develop-ment than in April before leaf emergence Rates of GPPgenerally followed PAR with highest rates on the cleardates in April and lowest rates in June in both streamsIn April rates of GPP were 4 to 53 higher in the WestFork than the East Fork despite slightly higher PARvalues in the East Fork on the same date because ofhigher algal and bryophyte biomass in the West ForkFor example in April 2000 average epilithon andbryophyte biomasses were 57 and 25 g AFDMm2 inthe West Fork compared with 07 and 06 g AFDMm2

in the East Fork (PJM unpublished data) Filamentousalgae also were visibly more abundant in the WestFork than the East Fork at this time (PJM personalobservation) The higher algal and bryophyte bio-masses in the West Fork were probably a result of a

2006] 587NO3

UPTAKE IN STREAMS

more stable benthic substratum in the West Fork InJune rates of GPP were very low in both streams butagain rates were higher in the West Fork than in theEast Fork despite nearly 23 higher PAR in the EastFork

April NO3 uptake rates

Both streams showed significant diurnal and day-to-day variations in NO3

uptake rate (k) as determinedfrom the longitudinal decline in tracer 15N flux (Fig1A B) These values of k corresponded to NO3

uptakelengths ranging from 112 to 310 m in the West Forkand from 61 to 83 m in the East Fork NO3

uptakelengths were shorter in the East Fork than in the WestFork largely because discharge and NO3

concentra-tions were lower in the East Fork (Table 1)

Despite similar light regimes (Fig 2A B) diurnaland day-to-day variations in NO3

uptake parameterswere considerably greater in the West Fork (Fig 2C EG) than in the East Fork (Fig 2D F H) in April In the

West Fork k was 2 to 33 greater during middayperiods (ranging from 00063ndash00090m) than duringthe predawn sampling (00032m) and k was 50greater on the 2 clear days (7 and 9 April) than on theovercast day (5 April) (Fig 2C) In addition k wasnearly 23 greater during the midnight sampling(00060m) than during the predawn period on thesame date Values of k also were significantly greateron the 2 clear days than at midnight although k on theovercast day did not differ from k for the previousmidnight The diurnal and day-to-day variations in kresulted in similar variations in Vf (Fig 2E) and U (Fig2G) because differences in stream discharge and NO3

concentration were small over the 4-d experimentalperiod (Table 1) Midday Vf (0090ndash0128 cmmin) andU (156ndash208 lg N m2 min1) values were 2 to 33greater than predawn Vf (0046 cmmin) and U (92 lgN m2 min1) values and the clear-day Vf (0125 and0128 cmmin) and U (191 and 208 lg N m2 min1)values were 50 higher than midnight Vf (0085 cmmin) and U (141 lg N m2 min1) values

TABLE 1 Stream characteristics during each of the 15N addition experiments in each stream

Stream Date (2001)Time ofsampling Period Discharge (Ls)

Watertemperature (8C)

NO3 concentration(lg NL)

West Fork 4 April 2345 Midnight 66 132 1665 April 0630 Predawn 65 131 2005 April 1430 Midday 65 140 1707 April 1430 Midday 65 168 1539 April 1415 Midday 58 174 16211 June 2345 Midnight 34 152 50512 June 0540 Predawn 34 150 49812 June 0950 Midmorning 34 152 46912 June 1345 Midday 33 164 475

East Fork 5 April 0050 Midnight 41 120 485 April 0710 Predawn 40 118 595 April 1440 Midday 40 126 457 April 1450 Midday 39 155 4612 June 0050 Midnight 06 170 46912 June 0600 Predawn 05 164 49612 June 1000 Midmorning 05 165 50812 June 1420 Midday 04 185 486

TABLE 2 Daily average water temperature photosynthetically active radiation (PAR) gross primary production (GPP) andecosystem respiration (R) on each of the dates of 15N addition experiments in each stream

Stream Date (2001)Water

temperature (8C)Daily PAR

(mol quanta m2 d1)Daily GPP

(g O2 m2 d1)Daily R

(g O2 m2 d1)

West Fork 5 April 131 50 20 227 April 139 120 47 479 April 145 110 51 3812 June 154 10 02 19

East Fork 5 April 121 51 05 527 April 130 154 09 6012 June 173 18 01 39


In the East Fork midday k values (0014 and 0016

m) were only 10 to 30 greater than midnight and

predawn k values (0012 and 0013m) but the

differences between midday and midnight values

were significant (Fig 2D) In addition k was signifi-

cantly greater on the clear day (7 April) than on the

overcast day (5 April) but again the difference was

relatively small (14) Values of k did not differ

between midnight and predawn in the East Fork

Diurnal and day-to-day variations in Vf (Fig 2F) and U

(Fig 2H) also were small because differences in stream

discharge and NO3 concentration (Table 1) were

minimal between sampling periods (Table 1) Midday

Vf values (0234 and 0267 cmmin) were only slightly

greater than the midnight and predawn Vf values

(0211 and 0200 cmmin) and midday U values (105

and 123 lg N m2 min1) were similar to the midnightand predawn U values (101 and 118 lg N m2 min1)

June NO3 uptake rates

Light regimes were similar in the West and EastForks (Fig 3A B) The magnitudes and diurnalvariations in NO3

uptake parameters differed be-tween streams (Fig 3CndashH) and differed from Aprilvalues in each stream In the West Fork k wassignificantly different from 0 only at midday (00014m Fig 3C) As in April diurnal patterns in Vf and U(Fig 3E G) were similar to diurnal patterns for kbecause of minimal differences in NO3

concentrationsand discharge between sampling periods (Table 1) Inthe West Fork daytime Vf and U values were 5 to 103greater than predawn values (Fig 3E G) but all valueswere low and error bars were large relative to themean suggesting that diurnal variations were notimportant in stream NO3

dynamicsIn the East Fork k values were 4 to 93 higher

(00055 to 0012m Fig 3D) than in the West Fork (Fig3C) and values differed significantly between sam-pling periods In the East Fork k was 23 higher atmidday than at midnight and predawn and mid-morning values of k were 50 greater than the nightvalues Vf and U were considerably greater in the EastFork (Fig 3F H) than in the West Fork (Fig 3E G) andEast Fork midmorning and midday values were 15to 23 greater than night values Although Vf wasconsiderably lower in June than in April in the EastFork daytime U was greater in June than in Aprilreflecting much higher NO3

concentrations in Junethan in April (Table 1)

Relationships between U and PAR

In April relationships between U and daily PAR(Fig 4A) and between U and daily GPP (Fig 4B) weresignificant for the West Fork but not for the East ForkThe intercepts of both relationships were similar (118lg N m2 min1) and represent dark NO3

demands ofheterotrophs and autotrophs The slopes of theserelationships (070 for U vs PAR and 168 for U vsGPP) represent the incremental daytime NO3

demandresulting from primary production If we convert theseslopes to equivalent units and assume a day length of12 h the incremental daytime NO3

uptake expressedper unit PAR was 10 3 103 lg Nlmol quanta andexpressed per unit of GPP was 24 3 103 lg Nlg O2If we assume a productivity quotient of 10 (mole CO2

consumedmole O2 produced) and net primaryproduction (NPP) of frac12 GPP then the incrementaldaytime NO3

demand per unit of autotrophic C

FIG 1 Regressions of ln(tracer 15N flux) vs distance in theWest Fork (A) and East Fork (B) of Walker Branch forexperiments during the period 5 April to 9 April 2001 Theslopes of each of the lines are the uptake rates (k) of NO3

perunit distance Regressions with significantly different (p

005) values of k have different letters (in parentheses next tolegend)

2006] 589NO3

UPTAKE IN STREAMS

synthesis was 0013 lg Nlg C (0011 on a molar basis)

in the West Fork This value is only 30 of the NC

ratio of West Fork periphyton measured in a previous

study at this time of year (Mulholland et al 2000)

suggesting that a considerable amount of the auto-

trophic demand for N is met by uptake of NO3 at

night or by uptake of other forms of N such as NH4thorn

Discussion

Diurnal and day-to-day variation

We showed that there were substantial diurnal andday-to-day variations in NO3

uptake related to lightlevel and primary productivity in the West Fork ofWalker Branch We presume that these differences in

FIG 2 Mean daily photosynthetically active radiation (PAR A B) NO3 uptake rate (k C D) uptake velocity (Vf E F) and

uptake (U G H) for the experiments conducted during April 2001 in the West and East Forks of Walker Branch Dark bars indicatenighttime measurements Bars for values of k with different letters are significantly different (p 005) among experiments (see Fig1) The error bars are upper 95 confidence intervals determined from the regressions for k (see Fig 1) and from error propagationfor calculations of Vf and U (see text and equations 7 and 8 for details) assuming no error in measurements of NO3

concentrationaverage water velocity average width and average water depth


NO3 uptake were the result of differences in the

demand for N and the ability of stream autotrophs

(primarily benthic algae and bryophytes) to use NO3

We predicted that light level would influence whole-

stream rates of NO3 uptake in seasons and streams

with relatively high rates of primary production

because photosynthesis provides additional energy

that can be used to reduce NO3 for use in metabolism

and biosynthesis (Gutschick 1981 Huppe and Turpin

1994) We found greater diurnal and day-to-day

variation in NO3 uptake rates in the West Fork than

in the East Fork in April and very low NO3 uptake

rates in the West Fork in June These results were

consistent with our prediction In early spring (before

leaf emergence) the West Fork has considerably higher

autotrophic biomass and rates of GPP than the East


uptake (U G H) for the experiments conducted during June 2001 in the West and East Forks of Walker Branch Dark bars indicatenighttime measurements No value for k differed from 0 in the West Fork on 11 to 12 June except the midday uptake rate whichwas marginally significant (p frac14 0052 denoted by the asterisk) Bars for values of k in the East Fork with different letters aresignificantly different (p 005) among experiments (see Fig 1) Error bars are as in Fig 2

2006] 591NO3

UPTAKE IN STREAMS

Fork but in late spring (after leaf emergence) GPP

drops to very low levels in both streams Analyses of

long-term NO3 concentration data from the West Fork

indicate a consistent spring peak in instream NO3

uptake (Mulholland and Hill 1997 Mulholland 2004)

An intensive study of stream NO3 concentration light

level and primary production in the West Fork during

spring showed increasing NO3 concentrations that

coincided with declining light levels and rates of

instream primary production as leaves emerged in the

forest canopy (Hill et al 2001) The significant diurnal

variation in NO3 uptake rate that occurred under low

light in the East Fork during June however was not in

agreement with our prediction and we have no

explanation for this result

The observation of lower streamwater NO3 con-

centrations at midday than at predawn in both theWest and East Forks in April (Table 1) also suggestsincreased NO3

uptake during the day We do not havemeasurements of NO3

concentration at sufficientfrequency to determine diurnal patterns in our currentstudy but a previous study in the West Fork of WalkerBranch (10ndash11 April 1991) indicated diurnal variationin NO3

concentrations of up to 14 lg NL or 50of the mean concentration on that date (Mulholland1992) Mulholland (1992) also observed diurnal varia-tion in dissolved organic C (DOC) concentrations thatwere opposite to those of NO3

concentrationsuggesting that stream autotrophs were taking upNO3

and releasing DOC at greater rates during theday than at night

Stream water temperatures also showed diurnalvariation in both streams particularly in April (Table1) but this variation does not appear to account for allof the increases in NO3

uptake rates observedbetween predawn and midday periods To estimatethe potential effect of variation in stream watertemperature we calculated increases in NO3

uptakerates from predawn values assuming a Q10 of 20 (iedoubling of uptake rate per 108C increase in temper-ature) In the West Fork 56 and 54 of the increasesin NO3

uptake rate between the 5 April predawn and7 April and 9 April midday measurements could beexplained by the 37 and 438C increases in watertemperature respectively In the East Fork 100 ofthe April predawn to midday increases and 75 of theJune predawn to midday increase in NO3

uptake ratecould be explained by water temperature increases

Other studies showing diurnal variation

Other researchers have reported diurnal variationsin NO3

concentrations in streams Manny and Wetzel(1973) reported diurnal variations in NO3

concen-tration of 200 lg NL in Augusta Creek Michiganin October but it was not clear if this variationreflected increases in the stream or in the marshes orlake upstream of the study site Grimm (1987) reporteddiurnal variations in NO3

concentration of nearly 100lg NL in Sycamore Creek Arizona a desert streamwith high GPP Burns (1998) observed diurnal varia-tions in NO3

concentration in 2 reaches of theNeversink River in the Catskill Mountains of NewYork throughout the spring and summer and attrib-uted these to autotrophic uptake within the stream

Fellows et al (2006) reported that NO3 uptake rates

were generally higher during the day than at nightduring summer in 2 forested steams in the southernAppalachian Mountains (one of which was the East

FIG 4 Relationships between NO3 uptake (U) and daily

photosynthetically active radiation (PAR A) and daily grossprimary production (GPP B) for the April experiments Theregressions for the West Fork data were statisticallysignificant (p 005) and are shown as lines in the plots


Fork of Walker Branch) and 2 forested streams in themountains of New Mexico Fellows et al (2006) alsoreported that NO3

uptake rates were higher in theNew Mexico streams (open forest canopies) wherelight levels were higher than in the Southern Appa-lachians (dense forest canopies) They used differentmethods (NO3

addition experiments conducted inbenthic chambers and in the stream) than we did butthey found diurnal variations in NO3

uptake rate thatare consistent with our observations However theNO3

uptake rates measured by Fellows et al (2006)were frac12 those reported in our study probablybecause of differences in methods between their studyand ours The nutrient-addition approach used byFellows et al (2006) yields lower estimates of nutrientuptake than tracer approaches (Mulholland et al 19902002 Payn et al 2005)

Nighttime variation

Algae can take up NO3 in the dark using stored C

reserves (Abrol et al 1983) but our results indicate thatalgal uptake is not constant through the night Weobserved significantly lower NO3

uptake rates justbefore dawn than near midnight in the West Fork inApril when algal production was high We suggestthat this result may have been caused by depletion ofstored photosynthate generated during daylighthours Thus there may be a 24-h cycle in NO3

uptakewith highest rates during the middle to latter part ofthe daylight period and lowest rates at night justbefore dawn

Role of autotrophs and GPP

Our results showing strong relationships betweenNO3

uptake rates PAR and GPP in the West Forkduring April indicate the important role of autotrophyin regulating N dynamics in streams Hall and Tank(2003) also have reported a significant relationshipbetween NO3

uptake expressed as Vf and GPP for 11streams in Grand Teton National Park Hall and Tank(2003) used a stoichiometric model that suggested thatstream autotrophs could account for most if not all ofthe NO3

uptake in the streams but they did notinvestigate diurnal variations in NO3

uptake ratesWe estimated expected periphyton uptake rates and

autotrophic demand for N based on periphyton NCstoichiometry a productivity quotient of 10 and NPPfrac14frac12(GPP) The low incremental daytime NO3

uptakerates for the rate of GPP observed in the West Fork inApril (30 of expected) suggests that a considerableamount of the autotrophic demand for N was met byuptake of NO3

at night or by uptake of other forms ofN (eg NH4

thorn) If the average nighttime NO3 uptake

rate in the West Fork in April (118 lg N m2 min1)represents a basal uptake rate of autotrophs then thisuptake would account for 40 of the total auto-trophic N demand resulting from NPP on 9 April(highest GPP in our study) Therefore it seems likelythat other forms of N also are important sources forautotrophs

Interactions with NH4thorn

NH4thorn concentrations in the West Fork during the

April experiments (2ndash5 lg NL) were considerablylower than NO3

concentrations but in a previousstudy Mulholland et al (2000) measured high rates ofNH4

thorn uptake at low NH4thorn concentrations using the

tracer 15N addition approach In April 1997 Mulhol-land et al (2000) measured an NH4

thorn uptake rate of 22lg N m2 min1 when NH4

thorn and NO3 concentrations

were similar to those in our current study suggestingthat NH4

thorn uptake could have accounted for asignificant portion of the autotrophic demand for Nin our current study

In the earlier study which involved a 6-wk tracer15NH4

thorn addition to the West Fork Mulholland et al(2000) observed a substantial interaction betweenNH4

thorn concentration and light level in regulatingNO3

uptake Mulholland et al (2000) calculatednitrification and NO3

uptake rates from the longi-tudinal distribution of tracer 15NO3

generated duringthe 15NH4

thorn addition and reported that NO3 uptake

rates declined from 288 lg N m2 min1 on a clear daybefore leaf emergence (1 April 1997) to 9 lg N m2

min1 after leaf emergence (12 May 1997) Thesevalues are similar to values measured in our currentstudy in the West Fork at midday before (7 and 9April Fig 2G) and after leaf emergence (12 June Fig3G) However Mulholland et al (2000) also reportedthat NO3

uptake rates were undetectable on 20 April1997 under partial leaf emergence when NH4

thorn

concentrations were 23 greater than NH4thorn concen-

trations measured in the April experiment of ourcurrent study

Implications for stream nutrient dynamics

Our results showing light-driven diurnal and day-to-day variations in NO3

uptake point to thepotentially important role of autotrophs in nutrientuptake in forested streams particularly during seasonswhen deciduous vegetation is dormant and light levelsare relatively high Our results for the West Forkshowing relatively high NO3

uptake in April andminimal uptake in June suggest that nearly all NO3

uptake in April was by autotrophs In the East Forkautotrophs appeared to play only a minor role in NO3

2006] 593NO3

UPTAKE IN STREAMS

uptake even under relatively high light levels in AprilHowever this result was consistent with the low ratesof GPP measured in the East Fork where abundance ofautotrophs was considerably lower than in the WestFork presumably because the substratum in the EastFork was less stable (gravel and fine-grained sedi-ments) than in the West Fork (bedrock and largecobble) Thus our results suggest that substratumcharacteristics may play an important role in control-ling the seasonal importance of autotrophs and theirimpact on nutrient cycling in forest streams

Some time ago Minshall (1978) pointed out theimportance of autotrophy in regulating stream ecosys-tem structure and function even for streams drainingforested catchments that might be considered netheterotrophic over an annual period Minshall (1978)noted the modeling study by McIntire (1973) showingthat low algal standing stocks in streams can lead tothe erroneous assumption that autotrophy is relativelyunimportant McIntire (1973) and Minshall (1978)showed that high algal productivity and turnoverrates can support relatively large populations ofprimary consumers and rates of secondary productiondespite low algal standing crops Our results indicat-ing that autotrophs are important in controllingnutrient dynamics in the West Fork of Walker Branchlend support to Minshallrsquos (1978) argument thatautotrophy can be an important regulating factor inforested stream ecosystems

Our results showing diurnal and day-to-day varia-tion in NO3

uptake have important implications forlonger-term assessments of N cycling in streamsMeasurements of NO3

uptake usually are madeduring the day and may overestimate uptake whenextrapolated to a 24-h period in streams with relativelyhigh levels of autotrophy In addition NO3

uptakerates measured on relatively clear days may not berepresentative of rates on overcast days Last rates anddiurnal variation in NO3

uptake may be much greaterin late winter and spring than at other times in streamsdraining deciduous forests and annual estimates ofNO3

uptake must account for these seasonal effects

Acknowledgements

We thank Ramie Wilkerson for laboratory analysisof water samples and Bill Mark EnvironmentalIsotope Laboratory University of Waterloo WaterlooOntario for the 15N analysis We thank Brian RobertsMichelle Baker and 2 anonymous referees for theirhelpful comments on earlier versions of the manu-script This work was supported by a grant from theUS National Science Foundation (DEB-9815868)

Literature Cited

ABROL Y P S K SAWHNEY AND M S NAIK 1983 Light anddark assimilation of nitrate in plants Plant Cell Environ-ment 6595ndash599

ALEXANDER R B R A SMITH AND G E SCHWARZ 2000 Effectof stream channel size on the delivery of nitrogen to theGulf of Mexico Nature 403758ndash761

APHA (AMERICAN PUBLIC HEALTH ASSOCIATION) 1992 Stand-ard methods for the examination of water and waste-water American Public Health Association AmericanWaterworks Association and Water Environment Fed-eration Washington DC

BOTT T L J T BROCK C S DUNN R J NAIMAN R W OVINKAND R C PETERSEN 1985 Benthic community metabolismin four temperate stream systems an inter-biomecomparison and evaluation of the river continuumconcept Hydrobiologia 1233ndash45

BURNS D A 1998 Retention of NO3 in an upland stream

environment a mass balance approach Biogeochemistry4073ndash96

FELLOWS C S H M VALETT C N DAHM P J MULHOLLANDAND S A THOMAS 2006 Coupling nutrient uptake andenergy flow in headwater streams Ecosystems (in press)

GRIMM N B 1987 Nitrogen dynamics during succession in adesert stream Ecology 681157ndash1170

GUTSCHICK V P 1981 Evolved strategies in nitrogenacquisition by plants American Naturalist 118607ndash637

HALL R O AND J L TANK 2003 Ecosystem metabolismcontrols nitrogen uptake in streams in Grand TetonNational Park Wyoming Limnology and Oceanography481120ndash1128

HILL W R P J MULHOLLAND AND E R MARZOLF 2001Stream ecosystem responses to forest leaf emergence inspring Ecology 822306ndash2319

HILL W R M G RYON AND E M SCHILLING 1995 Lightlimitation in a stream ecosystem responses by primaryproducers and consumers Ecology 761297ndash1309

HUPPE H C AND D H TURPIN 1994 Integration of carbonand nitrogen metabolism in plant and algal cells AnnualReview of Plant Physiology and Plant Molecular Biology45577ndash607

JOHNSON D W AND R I VAN HOOK 1989 Analysis ofbiogeochemical cycling processes in Walker BranchWatershed SpringerndashVerlag New York

LAMBERTI G A 1996 The niche of benthic algae in freshwaterecosystems Pages 533ndash572 in R J Stevenson M LBothwell and R L Lowe (editors) Algal ecologyfreshwater benthic ecosystems Academic Press SanDiego California

MANNY B A AND R G WETZEL 1973 Diurnal changes indissolved organic and inorganic carbon and nitrogen in ahardwater stream Freshwater Biology 331ndash43

MARZOLF E R P J MULHOLLAND AND A D STEINMAN 1994Improvements to the diurnal upstream-downstreamdissolved oxygen change technique for determiningwhole-stream metabolism in small streams CanadianJournal of Fisheries and Aquatic Sciences 511591ndash1599

MCINTIRE C D 1973 Periphyton dynamics in laboratory


streams a simulation model and its implicationsEcological Monographs 43399ndash420

MCKNIGHT D M R L RUNKEL C M TATE J H DUFF AND DL MOORHEAD 2004 Inorganic N and P dynamics ofAntarctic glacial meltwater streams as controlled byhyporheic exchange and benthic autotrophic commun-ities Journal of the North American BenthologicalSociety 23171ndash188

MINSHALL G W 1978 Autotrophy in stream ecosystemsBioScience 28767ndash771

MULHOLLAND P J 1992 Regulation of nutrient concentrationsin a temperate forest stream roles of upland riparianand instream processes Limnology and Oceanography371512ndash1526

MULHOLLAND P J 2004 The importance of in-stream uptakefor regulating stream concentrations and outputs of Nand P from a forested watershed evidence from long-term chemistry records for Walker Branch WatershedBiogeochemistry 70403ndash426

MULHOLLAND P J AND W R HILL 1997 Seasonal patterns instreamwater nutrient and dissolved organic carbonconcentrations separating catchment flow path and in-stream effects Water Resources Research 331297ndash1306

MULHOLLAND P J A D STEINMAN AND J W ELWOOD 1990Measurement of phosphorus uptake length in streamscomparison of radiotracer and stable PO4 releasesCanadian Journal of Fisheries and Aquatic Sciences 472351ndash2357

MULHOLLAND P J J L TANK D M SANZONE W MWOLLHEIM B J PETERSON J R WEBSTER AND J L MEYER2000 Nitrogen cycling in a forest stream determined by a15N tracer addition Ecological Monographs 70471ndash493

MULHOLLAND P J J L TANK J R WEBSTER W B BOWDEN WK DODDS S V GREGORY N B GRIMM S K HAMILTON SL JOHNSON E MARTI W H MCDOWELL J MERRIAM J LMEYER B J PETERSON H M VALETT AND W M WOLLHEIM2002 Can uptake length in streams be determined bynutrient addition experiments Results from an inter-biome comparison study Journal of the North AmericanBenthological Society 21544ndash560

MULHOLLAND P J H M VALETT J R WEBSTER S A THOMASL N COOPER S K HAMILTON AND B J PETERSON 2004Stream denitrification and total nitrate uptake ratesmeasured using a field 15N isotope tracer approachLimnology and Oceanography 49809ndash820

NEWBOLD J D J W ELWOOD R V OrsquoNEILL AND W VAN

WINKLE 1981 Measuring nutrient spiraling in streams

Canadian Journal of Fisheries and Aquatic Sciences 38

860ndash863

PAYN R A J R WEBSTER P J MULHOLLAND H M VALETT AND

W K DODDS 2005 Estimation of stream nutrient uptake

from nutrient addition experiments Limnology and

Oceanography Methods 3174ndash182

PETERSON B J W M WOLLHEIM P J MULHOLLAND J R

WEBSTER J L MEYER J L TANK E MARTI W B BOWDEN

H M VALETT A E HERSHEY W H MCDOWELL W K

DODDS S K HAMILTON S GREGORY AND D J MORRALL

2001 Control of nitrogen export from watersheds by

headwater streams Science 29286ndash90

RATHBUN R E D W STEPHENS D J SCHULTZ AND D Y TAI

1978 Laboratory studies of gas tracers for reaeration

Proceedings of the American Society of Civil Engineering

104215ndash229

SABATER F A BUTTURINI E MARTI I MUNOZ A ROMANI J

WRAY AND S SABATER 2000 Effects of riparian vegetation

removal on nutrient retention in a Mediterranean stream

Journal of the North American Benthological Society 19

609ndash620

SIGMAN D M M A ALTABET R MICHENER D C MCCORKLE

B FRY AND R M HOLMES 1997 Natural abundance-level

measurement of the nitrogen isotopic composition of

oceanic nitrate an adaptation of the ammonia diffusion

method Marine Chemistry 57227ndash242

STREAM SOLUTE WORKSHOP 1990 Concepts and methods for

assessing solute dynamics in stream ecosystems Journal

of the North American Benthological Society 995ndash119

VITOUSEK P M J D ABER R W HOWARTH G E LIKENS P A

MATSON D W SCHINDLER W H SCHLESINGER AND D G

TILMAN 1997 Human alteration of the global nitrogen

cycle sources and consequences Ecological Applications

7737ndash750

YOUNG R G AND A D HURYN 1998 Comment improve-

ments to the diurnal upstream-downstream dissolved

oxygen change technique for determining whole-stream

metabolism in small streams Canadian Journal of

Fisheries and Aquatic Sciences 551784ndash1785

Received 9 August 2005Accepted 16 March 2006

2006] 595NO3

UPTAKE IN STREAMS

J N Am Benthol Soc 2006 25(3)583ndash595 2006 by The North American Benthological Society

Effects of light on NO3 uptake in small forested streams diurnal

and day-to-day variations

Patrick J Mulholland1

Environmental Sciences Division Oak Ridge National Laboratory PO Box 2008 Oak RidgeTennessee 37831 USA

Steven A Thomas2 H Maurice Valett3 AND Jackson R Webster4

Department of Biology Virginia Polytechnic Institute and State University Blacksburg Virginia 24061 USA

Jake Beaulieu5

Department of Biological Sciences University of Notre Dame Notre Dame Indiana 46556 USA

Abstract We investigated the effects of autotrophy on short-term variations in nutrient dynamics bymeasuring diurnal and day-to-day variations in light level primary productivity and NO3

uptake duringearly and late spring in 2 forested streams the East and West Forks of Walker Branch in eastern TennesseeUSA We predicted that diurnal and day-to-day variations in NO3

uptake rate would be larger in the WestFork than in the East Fork in early spring because of higher rates of primary productivity resulting from amore stable substratum in the West Fork We also predicted minimal diurnal variations in both streams inlate spring after forest leaf emergence when light levels and primary productivity are uniformly low Reach-scale rates of gross primary production (GPP) were determined using the diurnal dissolved O2 changetechnique and reach-scale rates of NO3

uptake were determined by tracer 15N-NO3 additions In the West

Fork significant diurnal and day-to-day variations in NO3 uptake were related to variations in light level

and primary productivity in early spring but not in late spring consistent with our predictions In earlyspring West Fork NO3

uptake rates were 2 to 33 higher at midday than during predawn hours and 50higher on 2 clear days than on an overcast day several days earlier In the East Fork early spring rates ofGPP were 4 to 53 lower than in the West Fork and diurnal and day-to-day variations in NO3

uptake rateswere 30 considerably lower than in the West Fork However diurnal variations in NO3

uptake rateswere greater in late spring in the East Fork possibly because of diurnal variation in water temperature Ourresults indicate the important role of autotrophs in nutrient uptake in some forested streams particularlyduring seasons when forest vegetation is dormant and light levels are relatively high Our results also haveimportant implications for longer-term assessments of N cycling in streams that rely on daytimemeasurements or measurements only under limited weather conditions (ie clear days)

Key words nitrate uptake light diurnal patterns tracer 15N gross primary production nutrientspiraling

The role of autotrophs in the structure and function-

ing of streams has been an important topic in stream

ecology for some time In his seminal paper Minshall

(1978) argued that autotrophy is of primary impor-

tance in maintaining the structure and function of

many streams including those in forested catchments

that may have high rates of primary productivity for

only a few months each year In their interbiome

comparison of stream metabolism Bott et al (1985)

showed that rates of gross primary production (GPP)

exceeded respiration in one or more seasons in most

streams even those in forested regions

A number of studies have focused on the role of

autotrophy in total metabolism and support of the

food web in streams (Bott et al 1985 Hill et al 1995

1 E-mail address mulhollandpjornlgov2 Present address School of Natural Resources 309

Biochemistry Hall University of Nebraska LincolnNebraska 68583 USA E-mail sthomas5unledu

3 E-mail addresses mvalettvtedu4 jwebstervtedu5 jbeauliendedu

583


3 (but notNH4














Study Sites



Methods

15N addition












Laboratory analyses


















2006] 585NO3

UPTAKE IN STREAMS


to NH4thorn




d15N frac14 RSAMPLE

RSTANDARD

1

3 1000 frac121





15N15Nthorn14N

frac14

d15N

1000thorn 1

3 00036765

1thorn d15N

1000thorn 1

3 00036765

frac122





frac12NO3 Nifrac123































U frac14 Fk

wfrac128









Results







2006] 587NO3

UPTAKE IN STREAMS






















(g O2 m2 d1)Daily R

(g O2 m2 d1)






































2006] 589NO3

UPTAKE IN STREAMS








Discussion


























2006] 591NO3

UPTAKE IN STREAMS












































Nighttime variation




































thorn








2006] 593NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS


3 (but notNH4














Study Sites



Methods

15N addition












Laboratory analyses


















2006] 585NO3

UPTAKE IN STREAMS


to NH4thorn




d15N frac14 RSAMPLE

RSTANDARD

1

3 1000 frac121





15N15Nthorn14N

frac14

d15N

1000thorn 1

3 00036765

1thorn d15N

1000thorn 1

3 00036765

frac122





frac12NO3 Nifrac123































U frac14 Fk

wfrac128









Results







2006] 587NO3

UPTAKE IN STREAMS






















(g O2 m2 d1)Daily R

(g O2 m2 d1)






































2006] 589NO3

UPTAKE IN STREAMS








Discussion


























2006] 591NO3

UPTAKE IN STREAMS












































Nighttime variation




































thorn








2006] 593NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS

Methods

15N addition












Laboratory analyses


















2006] 585NO3

UPTAKE IN STREAMS


to NH4thorn




d15N frac14 RSAMPLE

RSTANDARD

1

3 1000 frac121





15N15Nthorn14N

frac14

d15N

1000thorn 1

3 00036765

1thorn d15N

1000thorn 1

3 00036765

frac122





frac12NO3 Nifrac123































U frac14 Fk

wfrac128









Results







2006] 587NO3

UPTAKE IN STREAMS






















(g O2 m2 d1)Daily R

(g O2 m2 d1)






































2006] 589NO3

UPTAKE IN STREAMS








Discussion


























2006] 591NO3

UPTAKE IN STREAMS












































Nighttime variation




































thorn








2006] 593NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS


to NH4thorn




d15N frac14 RSAMPLE

RSTANDARD

1

3 1000 frac121





15N15Nthorn14N

frac14

d15N

1000thorn 1

3 00036765

1thorn d15N

1000thorn 1

3 00036765

frac122





frac12NO3 Nifrac123































U frac14 Fk

wfrac128









Results







2006] 587NO3

UPTAKE IN STREAMS






















(g O2 m2 d1)Daily R

(g O2 m2 d1)






































2006] 589NO3

UPTAKE IN STREAMS








Discussion


























2006] 591NO3

UPTAKE IN STREAMS












































Nighttime variation




































thorn








2006] 593NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS







U frac14 Fk

wfrac128









Results







2006] 587NO3

UPTAKE IN STREAMS






















(g O2 m2 d1)Daily R

(g O2 m2 d1)






































2006] 589NO3

UPTAKE IN STREAMS








Discussion


























2006] 591NO3

UPTAKE IN STREAMS












































Nighttime variation




































thorn








2006] 593NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS






















(g O2 m2 d1)Daily R

(g O2 m2 d1)






































2006] 589NO3

UPTAKE IN STREAMS








Discussion


























2006] 591NO3

UPTAKE IN STREAMS












































Nighttime variation




































thorn








2006] 593NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS



































2006] 589NO3

UPTAKE IN STREAMS








Discussion


























2006] 591NO3

UPTAKE IN STREAMS












































Nighttime variation




































thorn








2006] 593NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS








Discussion


























2006] 591NO3

UPTAKE IN STREAMS












































Nighttime variation




































thorn








2006] 593NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS



















2006] 591NO3

UPTAKE IN STREAMS












































Nighttime variation




































thorn








2006] 593NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS












































Nighttime variation




































thorn








2006] 593NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS






Nighttime variation




































thorn








2006] 593NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS









Acknowledgements


Literature Cited

































860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS














860ndash863














104215ndash229





609ndash620













7737ndash750







2006] 595NO3

UPTAKE IN STREAMS

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